The present invention relates to a system for the fixing of anodized aluminum.
Following the anodization of aluminum, an oxide layer forms on the surface of the metal; this layer is porous and therefore readily open to attack from extraneous substances that cause its deterioration. Accordingly, subsequent fixing is required. The traditional method of fixing formerly employed was to immerse the anodized items in boiling water containing salts of nickel, for a period of some 20...40 minutes duration. Being slow and costly, this method has been replaced latterly by a process that is quicker and consumes less energy, whereby the anodized items are immersed in a cold bath containing predominantly nickel fluoride dissolved at a rate of 5 grams/liter.
The cold method in its turn has certain drawbacks: not only is the cost of nickel fluoride relatively high, but the nickel fluoride solution is easily contaminated by external agents; this leads to a marked reduction in the quantity of fluorine ions in solution, and ultimately to substandard results in fixing.
Contamination occurs, for example, through the presence of calcium ions; these form fluorides which precipitate and steal fluorine ions from the solution. Also, the solution contains aluminum ions that form fluoraluminate compounds, the effect of which is to render the fluorine ions unusable. Such, by way of example, are the principal sources of contamination to which the fixing solution is subject.
One expedient adopted in order to eliminate these difficulties consists in adding ammonia to induce precipitation of the aluminum and successively eliminate the precipitate, aluminum hydroxide, from the solution; the effectiveness of this method is offset by the drawback of its being long and laborious.
The object of the invention is to overcome the aforedescribed drawbacks through the provision of a system by means of which anodized aluminum fixing solutions can be purified simply, swiftly and economically, while maintaining the concentration of fluorine ions at prescribed levels.